Water Hammer

Water hammer is a temporary change in pressure in a pipeline due to a change in the velocity of flow in a pipe with respect to time, e.g. a valve opens or closes or a pump starts or stops. Accidental events such as a pipe blockage can also be a cause. The effects are exacerbated by:

fast closing/stopping valves/pumps

high water velocities

air in the line

poor layout of the pipe network, positioning of pumps, etc.

Note that water hammer pressure may be positive or negative. Both can be detrimental to pipe systems; not only pipes, but pumps, valves and thrust supports can be damaged. Negative pressures can cause ‘separation’ (vacuum formation), with very high positive pressures on ‘rejoinder’ (collapse of the vacuum). For these reasons, water hammer should be eliminated as far as possible.

It is beyond the scope of this manual to give a complete description of water hammer analysis and mitigation. However, it is appropriate to highlight some important aspects related to PVC pipes.

Celerity

Celerity is the speed (expressed in metres per second) that the pressure waves travel in a closed circuit. This should not be confused with the velocity of the water.

This is a function of the pipe geometry (dimension ratio) and material and may be estimated from:

where: W = density of fluid (water = 1,000) (kg/m3)
A = cross-sectional area of the wall of the pipe per unit length (mm2/mm)
= wall thickness for plain wall pipes
D = mean diameter of the pipe (mm)
k = the bulk modulus of the fluid (2150 for water) (MPa)
E = the elastic modulus for the pipe (MPa)

The wave celerity induced in PVC pipes are shown in Table 3-12. As PVC has a celerity about one third that of metallic pipes, analyses for metallic pipes should not be used to check PVC classes.

Dimension Ratio (DR) and Celerity (a)

PN

PVC-U

≤ DN 150

> DN 175

DR

a (m/s)

DR

a (m/s)

4.5

48.9

252

54.7

239

6

36.7

290

41

274

9

24.4

351

27.3

333

12

18.3

402

20.5

381

15

14.7

445

16.4

423

16

13.8

458

15.4

436

18

12.2

483

13.7

460

20

11.0

506

12.3

482

PVC-O All sizes

MRS/PN

DR

a (m/s)

400/12.5

40

309

500/16

40

309

500/20

32

344

PN

PVC-M All sizes

DR

a (m/s)

6

46

252

9

38.9

273

12

29.2

313

15

23.3

348

16

21.9

359

18

19.4

379

For buried pipes, increase the wave celerity (a) by 7%.

The advantage of a low celerity can be demonstrated by Joukowsky’s Law, which gives an estimate for the water hammer pressure rise due to instantaneous valve closure.

P = W a • ΔV (Pa)

where: ΔV = change in flow velocity (m/s)

This equation should NOT be used for design purposes. Water hammer analysis is fairly complex and computer analysis by a competent consultant is recommended wherever it is suspected that water hammer may be significant.

Pipe Response

Selection of class should be based on peak operating pressures including water hammer. Control devices may be useful in reducing peak pressures and enable a more economic pipe class to be used.

The response of the pipe to occasional abnormal pressures, for example due to the failure of protective devices, is important.

PVC has a high factor of safety on short term stress effects, and is able to withstand occasional events at higher than normal pressures. This advantage should be considered when determining the validity of basing a design purely on the pressures induced by events that may be rare in the design lifetime, e.g. power failure on a pump.